I don't think you're supposed to use pressure treated wood inside. Kiln dried is good though.
You did a great job for your first time. I guarantee that thing would hold up your car if you could get in on the stand.
You can trim it out with stock 1x4 or 1x6 boards and thin 1/4 inch plywood. You can add decorative molding as well. Then paint or stain. That is basically what I did with this one, except I used a router on the edges instead of attaching molding.